A well known type of container having a wide spread use today is the so called tearing strip container which is opened by means of a slotted key which engages and removes a portion of the circumference of the can that has been defined and weakened by score lines. It is the general practice to equip each such tearing strip with an end tab and cooperating key; the latter preferably secured to the top end wall of the container. It is to the novel technique by which this key is affixed to the can at high speed that this invention is directed.
Presently, the usual methods of affixing the key to a can wall are by spot welding or soldering of a portion of the key to subjacent metal of the can wall. However, certain disadvantages are associated with such processes. Due to the thin gauge of metal wall to which the key is attached the localized high temperature which burns through the metal can making same nonhermetic. This localized high temperature also has a tendency to char and degrade the metallic or organic coating on the inside and/or outside of the can wall, thereby exposing core metal which is subject to corrosion both from the atmosphere and from the contents packed in the container. Furthermore, soldering lead is not acceptable as a substance for use in food packaging. In addition, the common welding and soldering methods are not readily applicable to certain metals commonly used for can bodies and are inapplicable for nonmetallic containers.
In an effort to obviate the disadvantages of spot welding or soldering the keys to the container while retaining the advantages of these methods, i.e., speed of effecting the bond, strength of the bond, ease of key removal for use and low cost, attempts have been made to adhere the key to the can end by cements, resins, and hot melts.
Selection of the proper adhesive is essential, but difficult, since the environment to which the adhesive is subject is harsh and unfavorable to most adhesives. For example, vinyl polymers are sensitive to moisture, would degrade during autoclaving and would have inadequate adhesive strength. Similarly, rubber based cements are not only sensitive to water but they tend to age (cold flow) and discolor (yellow); they are also limited as to their operational temperature range. Thermosetting resins and rubbers require heat to cure and as such do not lend themselves to a high speed single step operation and also the latitude as to color (tending to yellow and brown) is limited with resin adhesives. Prior to the present invention, no commercially satisfactory way of quickly bonding the key to any type of container and any way of forming a bond which could be processed through, for example, autoclaving at elevated temperature with high moisture content has been known. Stacked cans with glued keys may stick to each other.
It is, therefore, an object of the present invention to provide a method of quickly attaching an opening key to a container wall without the danger of transmitting heat through the wall.
Still another object of the present invention is to provide a means of joining an opening key with a container wall without affecting adversely the contents of it.
A further object of the present invention is to provide an economical means of attaching an opening key to a can wall at a relatively high speed by means of specially selecting tape.